Observation of microearthquakes in the Atotsugawa fault region, central Honshu, Japan - Seismicity in the creeping section of the fault

摘要

Seismic activity along the Atotsugawa fault system, a prominent active fault system in the northwest Chubu district, Japan, indicates a linear trend along three fault traces of the fault system. The activity is not thought to be aftershocks of the large inland earthquake of M 7.0 that occurred in 1858, because of the long time lapse since the main shock. In addition, surveys with electro-optical distance-meters have indicated creep movements from the middle to the northeastern end of the fault. In order to clarify the relationship between the seismicity and fault movements, seven temporary seismic stations were established in and around the fault zone. Focal depths obtained from the new data are 2-3 km deeper than those determined. The deeper cutoff-depth of seismicity has a concave shape along the fault system. Moreover, few events have been located from the surface to 7-8 km into the upper crust, extending to beneath the creeping section of the fault, while seismicity is high on both sides of the low seismicity area. This hypo-center distribution suggests a heterogeneous stress state or asperities along the fault system. The change in the cutoff depth of seismicity along the fault is roughly concordant with the thermal structure, but the deepest cutoff depth is 16-18 km beneath the creeping section, which is deeper than usual. This may be caused by a high strain rate. GPS surveys show that displacement pattern changes along the fault system. This suggests that the fault is basically locked. Therefore, the fault system seems to be a shear zone and supports the global stress. Thus, the heterogeneous seismicity along the fault system seems to play an important role in the nucleation of large inland earthquakes.